docs: def export_ipynb(self, pathname):
          """
          Export notebook to .ipynb file
          :param pathname: output filename
          :return:
          """
  
          if pathname == '-':
              nbf.write(self.nb, sys.__stdout__)
          else:
              with codecs.open(pathname, 'w', encoding='utf-8') as f:
                  ret = nbf.write(self.nb, f)
                  pass
  
          logging.info("Jupyter notebook exported to '{}'".format(pathname))